I will say even more about ass and donkeys: "ass" in the UK still means ONLY "donkeys", and, as in Russian, either in the literal sense - a copper animal - or in the transportable, that is. "They are foolish" It comes from the Old English assa - Welsh asyn - Latin asinus.

"Butt" is "arse", a completely separate word with a completely separate origin — from the old English ærs, close to the Dutch aars and the German Arsch.

In American English in the 19th century, both words were mixed, and ass was used in speech, with the meaning being taken from arse. This is a historical coincidence that happened with many words.